Refiners are in need to modernize and optimize process such that the technology is equipped well enough to handle heavier crude baskets as the lighter crudes are getting depleted. This implies that the middle distillates fractionated are having reduced level of aliphatic hydrocarbons and higher level of aromatic hydrocarbons which indeed demands the need of Cetane improvement. The emission norms are becoming more stringent and to meet these standards, there is a need to improvise the cetane enhancement technology for betterment of diesel fuel quality. Cetane number (CN) has direct relation with the fuel quality and has vital impact on exhaust emissions. Cetane number is actually a measure of ignition quality of diesel fuel, i.e., fuel's ignition delay after injection and before combustion. Increasing Cetane number improves the ignition characteristics of fuel such as shorter ignition delays, easy cold weather starting, suppressing noise (engine knocking) during combustion, misfiring and improves engine efficiency. Higher Cetane also lowers the emissions, particularly NOx.
The cetane enhancement of the diesel range streams can be done broadly in two ways (1) by hydrogenation route and (2) by non hydrogenation route. The 1st route is much more obvious, established and being practiced extensively in the refinery. Still this route has got certain disadvantages, (a) it involves costly hydrogen (b) it is a very high pressure and high temperature process and hence cost intensive (c) hydrogen consumption of some streams like LCO are very high but still the cetane gain is very nominal. The 2nd route i.e. non-hydrogenation route can be classified in three categories viz. Cetane enhancement by (i) adding Additives, (ii) Nitration and (iii) adding Oxygen to the fuel. Additives like alkyl nitrate, di-tert-butyl peroxide, dimethyl ether and the like are used to enhance the CN but each additive has its own limitation either in way of quantity or cost. The nitration route gives very high CN boost, improves pour point, cloud point, and viscosity but it has very bad impact on stability. It increases sediment formation, haziness and results in deposits on storage. The third route is the addition of oxygen to the fuel.
The incorporation of oxygen in the diesel fuel improves its emission quality particularly NOx and particulate matter, and at the same time some oxygenate compounds also improves the burning quality i.e. cetane number of the fuel. For cetane improvement the oxidation reaction should be selective in nature otherwise the cetane may get deteriorated.
Various inventive works has been carried out in the past at improving cetane number of diesel range fuels. U.S. Pat. No. 4,494,961 provides method for increasing the Cetane number of a low hydrogen content, highly aromatic distillate through partial catalytic oxidation. Catalyst may be transition metal oxides alone or along with alkali/alkaline earth metal. European Pat. No. 0,252,606 discloses the Cetane improvement of middle distillate by catalytic oxidation at benzylic carbon atom using oxygen or oxidant at temperature below 200° C. with non-oxide metal compound catalyst. Also U.S. Pat. No. 4,723,963 discloses the additional information that diesel having at least 10 wt % alkyl aromatics or hydroaromatics can be selectively oxidized, preferably in to ketones.
European Pat. No. 0,293,069 discloses an additive tetralinhydroperoxide and use of the same as cetane improver such that peroxide number of diesel blend is 100-1000. It acts as cetane improver additive for diesel engine fuels and fuel production process by partial oxidation of hydrogenated diesel fraction originally having high aromatics (tetralin content>0.5 wt %) till the product reaches the peroxide number level of 100-150.
The method of manufacturing oxygenated fuel by contacting the feedstock with oxygen containing gas at oxidation conditions in presence of Group VIII metal catalyst on basic support is disclosed in U.S. Pat. No. 7,300,568. U.S. Pat. No. 7,501,054 discloses the process of upgrading diesel fuel by hydrogenating a portion of feedstock to enrich alkyl-naphthene-aromatic compounds followed by selective catalytic oxidation to alkyl ketones.
PCT application WO 2012/027820 discloses a method of cetane improvement by contacting diesel with ozone gas in presence of an alcohol and non-alcohol polar solvent (both solvents<10 vol %) to produce ozonated diesel oil along with oxidized byproducts which are removed later. Ultrasonic mixing of liquid hydrocarbon with oxidation source, catalyst and acids yields a diesel of substantially increased Cetane number. The cavitation created by ultrasonic mixing results in formation and collapse of micro-sized bubbles which highly increases the reactivity of reactants, is embodied in US Application. No. 2011/0065969.
In light of the above there still exists a need for an improved process for enhancing the quality of diesel fuels such as by enhancing cetane number.